The mechanism of the total synthesis of acetate from CO2 by Clostridium thermoaceticum and by Clostridium acidiurici is being investigated. The synthesis by C. thermoaceticum involves transmethylation from methyltetrahydrofolate to the cobalt of a corrinoid enzyme and carboxylation of the resulting methyl corrinoid by transcarboxylation from pyruvate. Five protein fractions have been obtained, which when combined, catalyze the synthesis of two acetyl phosphates from one pyruvate and one CH3-tetrahydrofolate. The proteins consist of ferredoxin, phosphotransacetylase (F1), transmethylase (F2), pyruvate decarboxylase (F4) and F3 a fraction containing 3 or 4 components, one of which is CO dehydrogenase and the second is almost certainly the corrinoid enzyme. Cofactor requirements are ATP, CoA and thiamin pyrophosphate (TPP). If the phosphotransacetylase is omitted and CoA is added in substrate amounts, the product is acetyl-CoA. Fraction F3 plus the transmethylase catalyzes the synthesis of acetyl-CoA from CO, CH3-tetrahydrofolate and CoA and ATP is required but not TPP. The details of the mechanism are under study and particularly the purification of the protein components of fraction F3. C. acidiurici synthesizes acetate from CO2 by a non-corrinoid pathway. This synthesis involves glycine decarboxylase and the mechanism of this synthesis is being investigated.